This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the correction of spatial distortion in magnetic resonance images due to magnetic susceptibility variations in an object being imaged.
Magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
The term "stereotaxis" refers to a collection of neurosurgical techniques that apply simple geometric relationships to radiological studies thereby facilitating the treatment of certain disorders of the brain with great accuracy. Over the past decade computed tomography (CT) has been the primary mode of target localization in stereotaxic surgery. However with the advent of the high resolution, high SNR, fast MR scanners, MR is more frequently applied to stereotaxic surgery because of its superior soft-tissue contrast.
Although Magnetic Resonance Imaging has proved to be useful in diagnostic radiology, it has yet to prove its utility in therapy for two major reasons. First, the geometric distortion associated with MR registration is quite significant. Second, present MR techniques are excellent in imaging soft-tissue and fat regions; but fail to image bony areas. In planning some surgical cases, the lack of bony detail in MR images is a disadvantage. Thus CT and MR provide useful information complementary to each other. As a result, there have been many attempts to combine the two modalities. Most efforts in multimodal image merging including MR have ignored the geometric distortion inherent to MR.
Misregistration due to magnetic susceptibility differences is by far the most complex source of error. It is dependent on both the material present in the imaging volume and the shapes of the structures being imaged. Usually such structures consist of inhomogeneous and anisotropic material. Although there have been recent attempts to characterize the fields inside the head by finite element analysis of Maxwell's equations, in general it is difficult to predict the amount of distortion accurately using such schemes. The magnetic properties of the human body are difficult to estimate and vary considerably depending on the factors such as the water content. Cho et al., "The Total Inhomogeneity Correction Including Chemical Shift and Susceptibility by View Angle Tilting," Medical Physics, Vol. 15, Jan./Feb. 1988, have proposed a clever method to correct the susceptibility distortion using view angle tilting by adding a compensation gradient. In practice however, difficulties arise due to bandwidth limitations of the gradient amplifiers. In addition, Chang and Fitzpatrick, "Geometric Image Transformation to Compensate for MRI Distortions," SPIE Medical Imaging: Image Processing, Vol. 1233, pp. 116-127, 1990, have presented a differential equation approach to correction of MRdistortion with the assumption of C.degree. continuity in the image profile.